As far as I have been lead to believe Medicine Buddha day is every "last quarter".
New moon day is usually Shakyamuni Buddha day, but hell, if you celebrate Sangye Menla yah can't go wrong! Essentially, all Buddhas, have one essence.
"Meditation is familiarisation with realisation"
Jigten Sumgon Gonchig: The Single Intent, the Sacred Dharma
"Oh great bodhisattva, you ought to understand the quintessence in this way: Whatever appears is one in its suchness. It cannot be falsified by anyone. The sovereign of unconceptualised sameness dwells in the spirit of the Dharmakaya which cannot be cognised."
The All Creating Sovereign, Mind of Perfect Purity.